1. Field of the Invention
The present invention relates to an apparatus for detecting a present driving azimuth of a vehicle and, more particularly, to a navigation apparatus capable of detecting an accurate driving azimuth of a vehicle without being adversely influenced by a disturbed geomagnetism or terrestrial magnetism.
2. Description of the Related Art
Various types of driving azimuth (direction) detecting apparatuses for vehicles have been known. For instance, there has been known an apparatus disclosed in Japanese KOKAI patent application No. 58-34483 opened on Feb. 28, 1983.
In such a known azimuth detecting apparatus, a present driving azimuth of a vehicle is detected by using a combination of a geomagnetic sensor which can detect an absolute azimuth of a vehicle and a gyroscopic sensor which can detect a variation in a relative azimuth of the vehicle.
The above-described conventional azimuth detecting apparatus will now be described with reference to FIG. 1. In a case where a vehicle is driven on, for instance, "a straight road" under a disturbed geomagnetic field condition, assuming that an actual driving azimuth of the vehicle is set to "P", the gyroscopic azimuth detected by the gyroscopic sensor corresponds to a stable straight line "Q" since it is not adversely influenced by the geomagnetic fields around the vehicle. However, in FIG. 1, the straight line "Q" is slightly deviated from the actual azimuth "P" because of the influence of a zero-point drift.
On the other hand, in the above azimuth detecting apparatus, the geomagnetic azimuth detected by the geomagnetic sensor is adversely influenced by such circumferential geomagnetic fields and fluctuates while drawing a sine curve "R".
Therefore, if such a geomagnetic azimuth is directly used as a vehicle driving azimuth in response to an output of the geomagnetic sensor, an error (indicated by "L" in FIG. 1) between the geomagnetic azimuth and the actual azimuth "P" becomes very large.
Therefore, in the azimuth detecting apparatus, obtaining a difference between the absolute azimuth "R" detected by the geomagnetic sensor and the relative azimuth "Q" detected by the gyroscopic sensor, enables two threshold levels "S.sub.1 " and "S.sub.2 " to be set. When the geomagnetic azimuth "R" detected by the geomagnetic sensor exceeds the threshold levels "S.sub.1 " and "S.sub.2 ", this azimuth "R" of the geomagnetic sensor is corrected so as to equalize the azimuth "R" to the threshold levels "S.sub.1 " and "S.sub.2 " and a corrected geomagnetic azimuth "T" is set to the present driving azimuth of the vehicle.
However, in such a conventional azimuth detecting apparatus, even if the error in the geomagnetic azimuth which is detected by the geomagnetic sensor is corrected and reduced to the threshold levels "S.sub.1 " and "S.sub.2 ", the large error ("L" in FIG. 1) with respect to the actual azimuth "P" still remains.
Therefore, in the case where the present location of the driving vehicle is calculated by using the above-described corrected geomagnetic azimuth as a reference, there is one problem such that the above-explained errors are accumulated.
In the foregoing conventional azimuth detecting apparatus, the gyroscopic sensor is used only to set the threshold levels and also is merely used to obtain the stable geomagnetic sensor output value. Thus, there is another problem such that the inherent merit of the gyroscopic sensor cannot be utilized effectively. That is, the relative azimuth variation amount can be accurately detected without being adversely influenced by the geomagnetic field condition.